The preparation of phosphorus-containing alkoxylation products, more particularly of organic phosphonates and halogen-substituted alkyl phosphates, is know to the skilled worker. Primarily phosphoric acid, phosphorous acid or phosphorus trihalide, preferably phosphorus trichloride, or phosphorus oxyhalide, more particularly phosphorus oxychloride, are used and are reacted with epoxides such as ethylene oxide, propylene oxide and/or epichlorohydrin. To increase the reaction rate it is common to use catalysts. For catalysts which operate homogeneously there are numerous versions known to the skilled worker.
Generally speaking, the alkoxylated products obtained have to be purified over a number of stages at different pH levels. The aftertreatment is usually accomplished by an aqueous workup of the crude reaction products, in the course of which the catalyst is destroyed irreversibly and is separated off. Aftertreatments of this kind for destroying or deactivating the catalyst, however, have drawbacks. They additionally necessitate reactors. There is a deterioration in the space/time yield, and losses of product occur. The washing water which is obtained must be disposed of, which is costly and inconvenient.
This is described, for example, in DD 125 035, where deactivation and/or destruction of the titanium halide catalyst employed therein is achieved by adding a stoichiometric amount of water or by scrubbing the phosphorus-containing alkoxylation products with water or alkalis.
The use of different catalysts in the reaction of phosphorus oxychloride with higher alkylene oxides such as propylene oxide generally results in a diastereomer mixture of the tris(chloropropyl) phosphates [referred to hereinbelow as TCPP]. In the course of this reaction the oxirane ring in propylene oxide is opened, with formation of TCPP. The choice of catalyst here has a critical influence on the ratio of the TCPP isomers (I) to (IV) to one another (see below). Furthermore, the choice of catalyst may also have a beneficial effect on the spectrum of by-products of the reaction, such as, for example chloropropanols, TCPP ethers (TCPP ethers=phosphates of I-IV with chlorinated polyether functions instead of chloropropyl), 2-methylpentenal, etc.
U.S. Pat. No. 3,100,220 discloses the use of titanium tetrachloride for preparing TCPP. This gives a TCPP having a specific density of 1.294. No composition of the isomers is disclosed.
DE-A 1 921 504 discloses a process for preparing chloroalkyl ester compounds with pentavalent phosphorus. A process is claimed for preparing chloroalkyl ester compounds with pentavalent phosphorus using aqueous titanium trichloride solutions in the form of a 0.001% by weight aqueous solution of TiCl3 that contains hydrochloric acid. No isomer composition of the TCPP is disclosed.
U.S. Pat. No. 2,610,978 describes the unadvantageous use of aluminium trichloride. It mentions the risk of an uncontrollable reaction between ethylene oxide and phosphorus oxychloride as a result of precipitating catalyst. The catalyst concentration has been disclosed at between 0.3% and 1% relative to the reaction product. A TCPP obtained from TiCl4 catalysis was described with a refractive index of nD30: 1.4608. No isomer distribution of the TCPP was specified.
EP 0 398 095 A2 discloses a process for preparing TCPP using titanium tetrachloride. The process is claimed for a phosphorus oxychloride/propylene oxide ratio of 1:3.09 to 1:3.50 mol/mol at 80-95° C. and a catalyst concentration of 1.7 to 2.8·10−3 mol/mol POCl3. The workup concerns a three-stage water scrub consisting of acid, neutral and alkali scrub with subsequent drying of the product. No precise product analysis of the TCPP is disclosed.
It was an object of the present invention, then, to develop a process for preparing phosphorus-containing alkoxylation products using a catalyst which ideally yields an optimized isomer ratio with as few secondary components as possible and hence with better yields. A low spectrum of secondary components, such as the TCPP ether content, for example, is critically important in respect of an advantageous toxicological evaluation. Furthermore, the process ought to operate with a minimum of scrubbing steps needed, and hence ought to be sparing in its use of resources.